Influence of seed inoculation and water regimes on soybean growth, yield, and water use efficiency

Brazil is the world’s largest soybean producer, yet yield is strongly limited by water deficit conditions. Strategies such as microbial inoculation and coinoculation may enhance plant tolerance to drought and improve water use efficiency. This study evaluated soybean responses to different seed inoculation and coinoculation strategies under contrasting water regimes, aiming to assess their potential to mitigate the effects of water limitation. A field experiment was conducted in Cachoeira do Sul, Brazil, using five inoculation treatments (non inoculated control, Bradyrhizobium japonicum , Azospirillum brasilense , a consortium of Bacillus subtilis , Bacillus amyloliquefaciens , and Trichoderma harzianum , and coinoculation with all microorganisms) and two water regimes (rainfed and supplemental irrigation). The cultivar Brasmax Zeus IPRO was used. Plant height, thousand grain mass, plant lodging, grain moisture, grain yield, and water use efficiency were evaluated. Water regime significantly affected all variables, while inoculation significantly influenced lodging, grain yield, and water use efficiency. A significant interaction between factors was observed only for lodging. Mean grain yield under rainfed conditions was 68.81 sacks per hectare, whereas supplemental irrigation increased yield to 102.00 sacks per hectare, a gain of 33.18 sacks per hectare (48%). Coinoculation with all microorganisms produced 110.08 sacks per hectare under irrigation, compared with 96.10 in the control (14.5% increase). Under rainfed conditions, coinoculation yielded 72.39 sacks per hectare versus 59.69 in the control (21.27% increase). Thousand grain mass was higher in rainfed plants (243 g) than irrigated ones (228 g). Water use efficiency reached 12.36 kg mm⁻¹ in inoculated rainfed treatments, compared with 10.06 kg mm⁻¹ in the control. The combined use of multiple beneficial microorganisms and supplemental irrigation enhances soybean productivity and water use efficiency under variable water availability.